Materials with a water activity value ranging from 0.90 – 0.85 are colonized by Aspergillus versicolor while those with water activity values of 0.85 or slightly less are colonized by Aspergillus versicolor, Eurotium spp., Wallemia spp., and Penicilliumspp., such as Penicillium chrysogenum and Penicillium aurantiogriseum.

Hazard Classes of Building Molds

The mold Penicillium growing on ceiling wood

In some countries building molds have been grouped into 3 hazard classes based on associated health risk. These classes are similar to risk groups assigned to microorganisms handled in laboratory environments.

Hazard Class A: includes fungi or their metabolic products that are highly hazardous to health. These fungi or metabolites should not be present in occupied dwellings. Presence of these fungi in occupied building requires immediate attention.

Hazard class B: includes those fungi which may cause allergic reactions to occupants if present indoors over a long period.

Hazard Class C: includes fungi not known to be a hazard to health. Growth of these fungi indoors, however, may cause economic damage and therefore should not be allowed.

Dr. Jackson Kung’u is a Microbiologist who has specialized in the field of mycology (the study of moulds and yeasts). He is a member of the Mycological Society of America. He graduated from the University of Kent at Canterbury, UK, with a Masters degree in Fungal Technology and a PhD in Microbiology. He has published several research papers in international scientific journals. Jackson has analyzed thousands of mould samples from across Canada. He also regularly teaches a course on how to recognize mould, perform effective sampling and interpret laboratory results. Jackson provides how-to advice on mould and bacteria issues. Get more information about indoor mould and bacteria at www.drjacksonkungu.com.